1. Technical Field
This invention described herein relates generally to the demodulation of digital signals. More particularly, the invention described herein relates to controlling a down conversion frequency to account for harmonics resident in a demodulator.
2. Related Art
Demodulation is a widely used process to make very high frequencies usable. One of the carrier recovery techniques in digital demodulation is based on the presence of a VCO (Voltage Controlled Oscillator) in the recovery loop. The VCO performs two functions: first, it searches for the carrier frequency in a procedure called a "frequency sweep" and second it tracks the carrier once the recovery loop locks. This is important as the frequency offset that it tracks might be up to +/-5 MHz. This frequency drift is mostly caused by low noise amplifier drift (LNA) or by other frequency conversion stages.
In real systems, the problem of a form of detrimental interference arises. One form of detrimental interference includes harmonic interference, also referred to as "harmonics". Harmonics are generally spectral components of a first signal which can interfere with carrier recovery in a carrier recovery loop. In particular, a major problem is the chance that oscillator harmonics are present in the neighborhood of a VCO. If the harmonics fall within the capture/ tracking range of the carrier recovery loop, there is a chance that (due to coupling) the VCO will become disturbed and either fail to lock or break lock once acquired. Measurements show that this interference could cause from 1 to 5 dB SNR loss. Specifically, this interference may be high enough in some cases to make the acquisition impossible (if the IF carrier falls over the harmonic) or to desynchronize the demodulator or the error correction stages following the demodulator (e.g., FEC decoding) (if tracking pulls the VCO frequency over the harmonic) especially when operating in a noisy channel. This problem also occurs in carrier recovery loops that do not include superheterodyning processes.
While increasing the operating specifications of devices used in circuit through the use of higher grade components or employing additional shielding around noisy components (via, for example, a metal shield or can enclosing the noisy device) may reduce the occurrence of detrimental harmonics, these steps fail to accommodate for the existence of the harmonics in sensitive frequency bands, whose mere occurrence may have detrimental effects. Accordingly, a solution is needed which accommodates real world harmonics while making the carrier recovery easioy accomplished, even when operating in a noisy channel.